1. Field of the Invention
The present invention generally relates to the field of implantable devices. More specifically, the present invention relates to an implantable medical device, for example, an implantable cardiac defibrillator for the identification of arrhythmias and the discrimination between non-lethal arrhythmias and lethal arrhythmias that may or may not require therapy.
2. Description of the Prior Art
Identifying an arrhythmia based on its intracardiac electrogram is a commonly used technique in, for example, implantable cardiac defibrillators (ICD), which are designed to deliver therapy for life-threatening ventricular arrhythmias. The heart rate is measured by detecting R waves in the IEGM originating from the ventricle using a ventricular lead. If the rate is higher than a specific limit, it is an indication that a tachycardia is present. A defibrillator is capable of delivering a high-energy electrical stimulus via medical leads to the heart that is sometimes referred to as defibrillation shocks. The shock interrupts a fibrillation, allowing the heart to re-establish a normal rhythm for efficient pumping of blood. The defibrillator is able to sense cardiac signals deliver therapy to heart based on such signals. However, a problem often encountered in these implantable cardiac defibrillators is that shocks are delivered inappropriately for e.g. supraventricular tachycardias. In fact, studies do indicate that roughly 20-30% of all delivered ICD shocks are inappropriate. About 16% of patients receive inappropriate therapies (single of multiple). It has further been found that patients experience 1.6 to 1.8 inappropriate shocks per year. One major reason for this delivery of inappropriate shock is the limited accuracy of the defibrillator in the discrimination between a truly life-threatening ventricular arrhythmia and a supraventricular tachycardia, which often causes similar electrical activity patterns, i.e. similar intracardiac electrograms. These inappropriate delivered shocks constitute a significant source of physical and emotional stress and discomfort for the patient. Further, the shocks also cause unnecessary consumption of the device battery, which shorten the life of the device.
Therefore, ICDs of today often use different complementary discrimination techniques. For example, the interval stability is a parameter that may be used to improve the discrimination since VTs (ventricular tachycardia) tend to be more stable in rate than SVTs (supraventricular tachycardias). Another parameter is atrial-ventricular event association. During sinus tachycardias (i.e. benign tachycardias caused e.g. of high physical activity) shows atrial-ventricular event association, e.g. each P wave is followed by an R wave. In case of VTs, the atrial and ventricular events are disassociated. Morphology analysis may also be used since it has been shown that the IEGM during a VT is different from that of a normal heart beat. SVTs give in most cases a rise to the normal morphology.\
A number of attempts have also been made within the prior art to overcome this problem.
For example, in U.S. Pat. No. 7,206,633 a device and method for identification and detection of abnormal heart rhythm occurring in either the supraventricular or ventricular cardiac regions and in particular a device and method for discriminating between supraventricular tachycardia and ventricular arrhythmia. The technique according to U.S. Pat. No. 7,206,633 is based on intracardiac electrograms recorded by atrial and ventricular sensing leads that distinguish their temporal relationships following tachycardia recurrence subsequent to a train of simultaneous anti-tachycardia pacing (ATP) bursts in the atria and ventricles.
In US 2005/0154421 an implantable medical device for identifying suspected non-lethal or lethal arrhythmias is shown. The device takes actions to avoid or delay delivery of a defibrillation shock in case of a non-lethal arrhythmia and respiration rate, respiration depth, and/or activity level is used to discriminate between lethal and non-lethal arrhythmia. Impedance measured over the thoracic region of the patient can be used to calculate the respiration rate and respiration depth. Thus, in US 2005/0154421, the respiratory pattern and/or activity level are used as additional input to the analysis of the intracardiac electrograms in the discrimination between non-lethal and lethal arrhythmias.
Furthermore, in U.S. Pat. No. 5,042,497 an implantable medical device capable of predicting and preventing cardiac arrhythmias is disclosed. The device monitors both ECG signals and autonomic neural tone signals. If both these signals indicate arrhythmia preventive or curative actions are taken by the device. Arrhythmias are characterized by increased sympathetic activity. This sympathetic activity can be followed by monitoring, among other, the respiration rate variability of the patient. According to U.S. Pat. No. 5,042,497, a decrease in variability of respiration rate indicate elevated sympathetic activity and thereby arrhythmia. The variability in respiration rate is determined on basis on the interval between consecutive breaths of the patient.
In U.S. Pat. No. 5,370,667 an implantable medical device that provides electrical therapy to a heart of a patient in order to treat pathological tachycardia is disclosed. The device has a first sensor that generates a signal indicative of the patient's ECG. A second sensor is an activity sensor that generates a signal representative of the current status of physical activity by the patient. For example, the impedance over the chest is measured to indicate the activity of the patient. The readings of the first sensor (activity) sensor are employed for dynamically adjusting the arrhythmia threshold used, together with the ECG signal, for detecting the presence of arrhythmia. The device utilizes different arrhythmia thresholds depending on the patient activity but the detection of the arrhythmia condition is performed solely based on ECG signals.
Consequently, a number of prior art solutions utilize breathing pattern as an additional parameter to the ECG signals to improve and increase the accuracy in the discrimination between lethal and non-lethal arrhythmias in order to reduce the number of erroneously delivered defibrillation shocks. However, there is still a need within the art of improved devices and method that are capable of discriminating between lethal and non-lethal arrhythmias with a higher degree of accuracy in order to further reduce the number of inappropriate delivered defibrillation shocks. As discussed above, such an improved device and method that is capable of reducing or even avoid inappropriate shock would have beneficial effect on the physical and emotional state of patients with defibrillators as well as reduce the cost of health care of the society.